1. Field of the Invention
The present invention relates in general to a tilt detector and tilt detection method for an optical head tilt servo device in an optical disc player.
2. Description of the Prior Art
An optical disc apparatus is generally adapted to project a small and marrow beam of light on an optical disc through an object lens to scan a fine mark (pit) on the disc so as to reproduce information recorded thereon and record new information thereon.
At this time, the surface of the optical disc may be tilted relative to a beam of light from an optical head due to a distortion of the disc, a surface vibration, etc. In this case, the beam of light from the optical head is incident on the surface of the optical disc to reproduce information recorded thereon, while being tilted relative to the disc surface, thereby making it difficult to accurately read the recorded information.
FIG. 7 shows variations in tilt of a spot of light formed on the surface of an optical disc by a beam of light, relative to the disc surface. In particular, the lower part of FIG. 7 shows the shapes of the light spot and the upper part thereof shows light intensity distributions of the spot.
In more detail, FIG. 7b shows the shape and intensity distribution of the light spot when the light beam is perpendicular to the disc surface. In this drawing, the light spot is shown to have a symmetrical shape. FIGS. 7a and 7c show the shapes and intensity distributions of the light spot when the disc surface is tilted relative to the light beam. In each of these drawings, a coma-aberration is generated in the light spot on the disc, and the light spot has an asymmetrical shape.
For the purpose of preventing the above coma-aberration, the optical disc apparatus comprises a tilt servo device for correcting a tilt of an optical axis of the optical head to maintain the optical axis perpendicular to the disc surface. The tilt servo device includes tilt detection means for detecting the amount of a tilt of a beam of light projected from the optical head relative to the disc surface.
A tilt sensor is generally used to detect a tilt amount. This tilt sensor is conventionally installed in the tilt detection means separately from an optical system which generates a beam of light for reproduction of information recorded on the optical disc. In this construction, the tilt sensor must be spaced apart from the object lens of the optical head at a certain distance in order to avoid an interference therebetween. However, the tilt sensor and the object lens of the optical head must become closer to each other to obtain a more approximate value to a tilt amount of a projected position of a light spot on the disc surface relative to the surface.
FIG. 8 is a perspective view showing an example of the construction of a head part in a conventional optical disc apparatus. A head part 102 is positioned under an optical disc 101, and an object lens 103 of an optical head and a tilt sensor 104 of tilt detection means are arranged on the upper surface of the head part 102. The tilt sensor 104 is arranged on the upper surface of the head part 102 in such a manner that a spot of light 106 formed by a beam of light 105 emitted from the object lens 103 passes therethrough and is substantially aligned therewith on a track 107 of the disc surface. Through this arrangement, the tilt sensor can approximately detect the position of the light spot 106 and the tilt amount of the disc surface.
However, in the head part with the above-mentioned construction, because the light spot and the tilt sensor are not perfectly aligned with each other, it is inevitable that a value detected by the tilt sensor is in error. This cannot guarantee a sufficient degree of precision, for example, when a high degree of tilt correction is required according to a recording densification of the disc.
Further, because the optical head and tilt detection means are provided separately, the number of components increases, resulting in an increase in cost and a complexity in the construction of the head part.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a tilt detector and tilt detection method which are capable of detecting a tilt in a radial direction of an optical disc at a high degree of precision and being cost-effective and simple in head part construction.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a tilt detector adapted to, by measuring a velocity of a spot position on a recording surface of an optical disc at which a beam of light is condensed, in a focusing direction of the optical disc and a velocity of the spot position in a rotation direction of the optical disc, calculate an angle defined by three vectors, or a focusing direction velocity vector in the focusing direction of the optical disc, a rotation direction velocity vector perpendicular to the focusing direction of the optical disc and a resultant vector of the focusing direction velocity vector and the rotation direction velocity vector and detect a tangential tilt angle on the basis of the calculated angle, the tangential tilt angle being a tilt angle in a tangential direction of the optical disc.
In accordance with another aspect of the present invention, there is provided a tilt detector for an optical disc information recording/reproduction apparatus which includes an optical head for projecting a condensed beam of light on a recording surface of a rotating optical disc and receiving the projected beam of light reflected from the recording surface of the optical disc, a focus actuator for focusing the condensed beam of light on its projected position of the recording surface of the optical disc, and focus servo means for controlling the focus actuator, the tilt detector comprising light beam direction velocity output means for outputting a signal indicative of a moving velocity of a contact point of the recording surface of the disc and the light beam in a direction depending on the light beam in response to a signal indicative of an operation amount of the focus actuator from the focus servo means; circumferential velocity output means for outputting a signal indicative of a relative tangential velocity between the light beam and the recording surface of the optical disc; and tilt angle calculation means for calculating a tilt angle of the light beam projected on the recording surface of the disc, relative to the recording surface in response to the output signal from the circumferential velocity output means and the output signal from the light beam direction velocity output means.
In accordance with a further aspect of the present invention, there is provided a tilt detector for an optical disc information recording/reproduction apparatus which includes an optical head for projecting a condensed beam of light on a recording surface of a rotating optical disc and receiving the projected beam of light reflected from the recording surface of the optical disc, a focus actuator for focusing the condensed beam of light on its projected position of the recording surface of the optical disc, focus servo means for controlling the focus actuator, and moving means for moving the projected position of the light beam on the disc with rotation of the disc, the tilt detector comprising light beam direction velocity output means for outputting a signal indicative of a moving velocity of a contact point of the recording surface of the disc and the light beam in a direction depending on the light beam in response to a signal from a certain point in the focus servo means; timing signal generation means for generating a timing signal in response to each rotation of the disc; storage means for storing the output signal from the light beam direction velocity output means in response to the timing signal from the timing signal generation means; and tilt angle calculation means for calculating a radial tilt angle of the light beam projected on the recording surface of the disc, relative to the recording surface in response to an output signal from the storage means, the radial tilt angle being a tilt angle in a radial direction of the disc.
Preferably, the focus servo means may include subtraction means for obtaining a difference between a target focusing value and a signal indicative of a current focusing state; and a compensator for outputting a signal indicative of an operation amount of the focus actuator in response to an output signal from the subtraction means; the light beam direction velocity output means being operated in response to the output signal from the subtraction means in the focus servo means.
Alternatively, the focus servo means may include subtraction means for obtaining a difference between a target focusing value and a signal indicative of a current focusing state; and a compensator for outputting a signal indicative of an operation amount of the focus actuator in response to an output signal from the subtraction means; the light beam direction velocity output means being operated in response to the output signal from the compensator in the focus servo means.
As an alternative, the focus servo means may include subtraction means for obtaining a difference between a target focusing value and a signal indicative of a current focusing state; and a compensator for outputting a signal indicative of an operation amount of the focus actuator in response to an output signal from the subtraction means; the light beam direction velocity output means being operated in response to the signal indicative of the current focusing state in the focus servo means.
Preferably, the tilt detector may further comprise prediction means for predicting a position of the contact point of the recording surface of the disc and the light beam in the direction depending on the light beam on the basis of the tilt angle calculated by the tilt angle calculation means.
In accordance with yet another aspect of the present invention, there is provided a tilt detection method for an optical disc information recording/reproduction apparatus which includes an optical head for projecting a condensed beam of light on a recording surface of a rotating optical disc and receiving the projected beam of light reflected from the recording surface of the optical disc, a focus actuator for focusing the condensed beam of light on its projected position of the recording surface of the optical disc, and focus servo means for controlling the focus actuator, the method comprising the steps of a) outputting a light beam direction velocity signal indicative of a moving velocity of a contact point of the recording surface of the disc and the light beam in a direction depending on the light beam in response to a signal indicative of an operation amount of the focus actuator from the focus servo means; b) outputting a circumferential velocity signal indicative of a relative tangential velocity between the light beam and the recording surface of the optical disc; and c) calculating a tilt angle of the light beam projected on the recording surface of the disc, relative to the recording surface in response to the circumferential velocity signal and the light beam direction velocity signal.